The invention relates to lasers, and more particularly to phase stabilization of mode locked lasers, and stable phase scanning of two lasers.
In many applications of mode-locked lasers, a pair of separate lasers are used in conjunction with one another and the output pulse trains must be stable in time with respect to each other or scan with respect to each other, in stable phase relationship. In order to phase stabilize the output pulse trains emitted from two separate mode locked lasers, for short and long term synchronization, a primary technique is to drive both mode lockers off the same radio frequency synthesizer or stable radio frequency (RF) source. This is accomplished, as is well known, using a power splitter to split the RF drive signal to serve both lasers. In addition, the two lasers must have the same cavity length or mirror spacing, with the RF frequency equal to the speed of light divided by four times the cavity length for acousto-optic modelocking, and two times the cavity length for phase modulation mode locking. Thus, with precisely the same cavity length in both lasers, and with both mode lockers of the two lasers driven from the same RF synthesizer, the two lasers should in theory be locked in simultaneous phase, stable with respect to each other.
However, an effect which may be called phase noise associated with both lasers will affect the timing between the pulses, so that it is not perfect. For example, there can be changes in cavity length of the lasers due to mechanical vibration or thermal fluctuations. As either laser has an effective change in cavity length, its phase relationship with respect to the driving source changes. Therefore, the two mode locked trains will drift in phase with respect to one another if either has a differential length change with respect to the other.
Similarly, a single laser's shifting in phase relationship with respect to its driving source can cause problems if subsequent timing signals are derived electronically from the RF source driving the mode locker.
One method of compensating for such cavity length changes is to monitor the optical output of the lasers with a fast photodiode, and then to servo the length of one laser with respect to the other to minimize the phase drift. This is discussed in Solid State Laser Engineering, W. Koechner, Springer-Verlag, New York 1976, at page 484. However, this method necessitates the use of a temperature sensitive element, such as a piezoelectric crystal driving a mirror, as an integral part of the cavity of one of the lasers, which considerably increases the sensitivity of the laser to thermal gradients. It also introduces a moving part, which adds to the complexity of the system.
No system of the prior art has been able to accomplish precise stabilization and control of the timing of a mode locked laser's pulse train and of stabilizing the relative timing of modelocked multiple lasers, without expensive and cumbersome mechanical apparatus, and no prior system has been able to accomplish rapid scanning of a pair of lasers while maintaining the lasers in completely stable phase relationship.